Climate is average weather, and it can vary unpredictably only within the limits set by major influences like the Sun and levels of greenhouse gases in the atmosphere. We might not be able to say whether it will rain at noon in a week’s time, but we can be confident that the summers will be hotter than winters for as long as the Earth’s axis remains tilted.

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The validity of models can be tested against climate history. If they can predict the past (which the best models are pretty good at) they are probably on the right track for predicting the future – and indeed have successfully done so.

Clouded judgement

Climate modellers may occasionally be seduced by the beauty of their constructions and put too much faith in them. Where the critics of the models are both wrong and illogical, however, is in assuming that the models must be biased towards alarmism – that is, greater climate change. It is just as likely that these models err on the side of caution.

Most modellers accept that despite constant improvements over more than half a century, there are problems. They acknowledge, for instance, that one of the largest uncertainties in their models is how clouds will respond to climate change. Their predictions, which they prefer to call scenarios, usually come with generous error bars. In an effort to be more rigorous, the most recent report of the IPCC has quantified degrees of doubt, defining terms like “likely” and “very likely” in terms of percentage probability.

Indeed, one recent study suggests that the feedbacks in climate systems means climate models will never be able to tell us exactly how much warming to expect. However, there is no doubt that there will be warming.

Given the complexity of our climate system, most scientists agree that models are the best way of making sense of that complexity. For all their failings, models are the best guide to the future that we have.

Finally, the claim is sometimes made that if computer models were any good, people would be using them to predict the stock market. Well, they are!

A lot of trading in the financial markets is already carried out by computers. Many base their decisions on fairly simple algorithms designed to exploit tiny profit margins, but others rely on more sophisticated long-term models.

Major financial institutions are investing huge amounts in automated trading systems, the proportion of trading carried out by computers is growing rapidly and a few individuals have made a fortune from them. The smart money is being bet on computer models.

Of course, in some ways financial markets are much trickier to model than the climate, depending as they do on human behaviour. What’s more, trading based on computer models alters the nature of the very thing you’re trying to predict.